It is well known in the prior art to prepare silicon nitride powder by directly nitriding metallic silicon powder in a reactive gas containing nitrogen or ammonia gas. Most such methods for preparing silicon nitride powder rely on a fluidized bed system wherein the reactive gas forms a fluidized bed with silicon nitride. A nitriding source material in the form of metallic silicon powder is continuously supplied into the fluidized bed where the source material is nitrided while the nitrided product is continuously removed from the fluidized bed for recovery.
These methods using the fluidized bed have a problem that since the fluidized bed is a perfect mixing system, the nitrided product is removed and recovered therefrom along with the unreacted source material or metallic silicon, resulting in a loss of degree of nitriding. There are several approaches for overcoming this problem, for example, by (1) increasing the reaction temperature, (2) extending the residence or reaction time of the source powder, (3) using multiple stages of fluidized bed (multi-stage reaction), and (4) treating the nitrided product with acid to remove the unreacted metallic silicon. Approaches (1) to (3), however, require an extended operating time of the associated reactor, leading to a substantial loss of throughput per unit time and unit power while it is imperative that the product be accompanied by some unreacted source material. Also, approach (4) gives rise to an economic problem because the acid treatment requires time and expense.
It was also proposed to use a moving bed reactor for nitriding metallic silicon powder with a reactive gas containing nitrogen or ammonia gas. The typical reactors used are a shaft kiln as disclosed in Japanese Patent Application Kokai No. 151311/1983, a tunnel kiln as disclosed in Japanese Patent Application Kokai No. 186406/1985, and a rotary kiln as disclosed in Japanese Patent Application Kokai No. 266305/1986. With respect to variation in quality such as .alpha. to .beta. phase ratio, degree of nitriding, and productivity, these methods using such moving bed reactor are advantageous in some aspects, but disadvantageous in other aspects, so that they are not fully adapted for manufacture on a commercial scale.